Electroplating of metals using mercapto-metal complex salts



United States Patent 3,238,112 ELECTROPLATIN G OF METALS USING MER-CAPTO-METAL COMPLEX SALTS John H. Haslam, Landenberg, P2,, assignor toE. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Filed July 3, 1962, Ser. No. 207,401

4 Claims. (Cl. 20445) {This invention relates to compositions andprocesses for the electrodeposition of metals. It is more particularlydirected to electroplating methods using baths containing mecaptocompounds.

It has been found that plating baths containing mercapto compounds havea number of advantages not possessed by conventional plating baths. Forexample, the need for using such highly toxic substances as cyanide inthe baths is eliminated. The baths give high efiiciency, both at theanode and at the cathode. They can be used at high pH levels, with theirattendant advantages. These baths have excellent throwing power, give ahigh quality plate, and have good conductivity. They also permit the useof copper and tin in their lower valence states, thereby saving platingcurrent.

The mercapto compounds which can be used according to this invention arethose of the formula where R is hydrogen, COOH, an alkyl radicalcontaining from 1 to 4 carbon atoms, or a hydroyalkyl radical containingfrom 1 to 5 carbon atoms;

where R is hydrogen, COOH, a hydroxyalkyl radical containing from 1 to 4carbon atoms, or an alkyl radical containing from 1 to 4 carbon atoms;and

n is 1 or 2.

Preferred for use are Z-mercaptoethanol Z-mercaptopropanoll-mercaptopropanol-Z Z-mercaptobutanol 2-mercaptobutanol-3l-thioglycerol 2 thioglycerol a-Thiosorbitol Mercaptoacetic acidZ-mercaptosuccinic acid Mercaptoethylamine The metals which can beelectrically deposited according to this invention are copper, tin,silver, gold, cadmium and zinc. Alloys of these metals, such as brass,can also be deposited. These metals can be deposited on such othermetals as steel, copper and brass.

The electroplating bath is prepared by dissolving a conventional amountof a salt of the metal to be deposited in an aqueous solution of amercapto compound which has been previously brought to pH l011 with astoichiometric amount of sodium or potassium hydroxide. The mercaptocompound should ordinarily be present at a concentration of from 2 tomoles per mole of the metal to be deposited. In the baths, the metals tobe deposited are naturally present as salts with the mercapto compound.Excess sodium or potassium hydroxide in the bath generally improvesplate quality and increases the conductivity of the bath. Conventionalbrightening 3,238,112 Patented Mar. 1, 1966 agents, such as Rochellesalts or sodium hydroxyacetate and other additives, can also be added ifdesired.

The plating is then carried out with conventional electr oplatingequipment, using ordinary electroplating techmques.

In order that this invention be better understood, the followingexamples are submitted:

Example 1 A solution of 45 grams of SnC1 -2H O, milliliters of water and10 milliliters of concentrated hydrochloric acid was prepared and thendiluted to 500 milliliters and adjusted to pH 7 with sodium hydroxide.The resulting precipitate of stannous hydroxide which formed wasfiltered under vacuum and washed several times with distilled water toremove sodium chloride.

This precipitate was dissolved in a solution of 75 grams of2-mercaptoethanol and 150 milliliters of water, previously adjusted topH 10 with sodium hydroxide.

This solution was then diluted to 267 milliliters with water and placedin a Hull cell. Five grams of sodium hydroxide were added to the bath.

Steel plates were used as a cathode and a tin plate was used as ananode. These plates were immersed in the bath and a current of 1 /2volts Was passed through.

The resulting tin plate was matte-white. Both cathode and anodeefliciency were These steel plates were then brightened by dipping theminto hot tallow, maintained at 207 C., for approximately 15 seconds.

i This method can also be used to plate silver and gold on brass orcopper by substituting 30 grams of Ag(NO or 20 grams of HAuCl -3H O forthe SnCl' -2H O. The tallow treatment is omitted.

Example 2 An electroplating bath was prepared by dissolving 15 grams ofcupr-ous oxide in a solution of 75 grams of mercaptoacetic acid inmilliliters of water, previously adjusted to pH 10 with sodiumhydroxide.

This solution was then placed in a Hull cell, 10 grams of sodiumhydroxide were added, and the resulting bath was used to electroplatecopper on steel, using a copper anode and a current of 2 /2 volts and 1ampere.

The resulting copper was bright and attractive over the whole currentdensity range.

Mercaptoethylamine can be substituted for mercaptoacetic acid in thisprocess, in equivalent amounts, with similar results.

Example 3 A solution of 20 grams of l-thioglycerol and 20 milliliters ofwater was adjusted to pH 10 with sodium hydroxide. Ten grams of cadmiumoxide were then added to this solution in small quantities and the wholewas made up to 267 milliliters with water.

This solution was then placed in a Hull cell, 5 grams of sodiumhydroxide were added, and the bath was electrolyzed using a steelcathode, a cadmium anode and a current of 0.5 ampere and 1 volt. Theresulting plate was bright and the anode and cathode platingefficiencies were essentially 100%.

Example 4 Eight grams of ZnO were dissolved in 200 milliliters of water,and sodium hydroxide was added until the pH of the solution was 9. Theresulting precipitate was filtered and washed with water.

This precipitate was then dissolved in a solution of 200 milliliters ofwater and 25 grams of Z-mercaptoethanol. The pH of this solution wasadjusted to 10 with sodium hydroxide and the solution was then made to267 milliliters with water.

This solution was used as an electroplating bath, using a steel cathode,a zinc anode and a current of 0.5 ampere and volts.

The resulting zinc plate was bright in the intermediate current densityrange.

The embodiments of the invention in which an exclusive property orprivilege is claimed are:

1. An improved electroplating bath consisting essentially of an aqueoussolution of a metal salt of a compound of the formula where R isselected from the group consisting of hydrogen, COOH, alkyl radicalscontaining from 1 to 4 carbon atoms, and hydroxyalkyl radicalscontaining from 1 to 5 carbon atoms; and

X is selected from the group consisting of (CHR),,OH,

(CHR'),,'NH and (OHR),, COOH where R' is selected from the groupconsisting of hydrogen, COOH, hydroxyalkyl radicals containing from 1 to4 carbon atoms and alkyl radicals containing from 1 to 4 carbon atoms;and

n is a whole positive number less than 3 the metal in said salt beingselected from the group consisting of tin, copper, silver, gold, cadmiumand zinc.

2. An electroplating bath consisting essentially of an aqueous solutionof a Z-mercaptoethanol salt of a metal selected from the groupconsisting of tin, copper, silver, gold, cadmium and zinc.

3. A method for the electrodeposition of tin, copper, silver, gold,cadmium and zinc, said method comprising passing an electric currentfrom a suitable anode to a cathode to be plated, through a bathconsisting essentially of an aqueous solution of a metal salt of acompound of the formula where R is selected from the group consisting ofhydrogen,

COOH, alkyl radicals containing from 1 to 4 carbon atoms, andhydroxyalkyl radicals containing from 1 to 5 carbon atoms; and

X is selected from the group consisting of (CHR'),,OH,

(CHRO NH and (CHR) COOH where R is selected from the group consisting ofhydrogen, COOH, hydr-oxyalkyl radicals containing from 1 to 4 carbonatoms and alkyl radicals containing from 1 to 4 carbon atoms; and

n is a whole positive number less than 3 the metal in said salt beingselected from the group consisting of tin, copper, silver, gold, cadmiumand zinc.

4. A method for the electrodeposition of tin, copper, silver, gold,cadmium and Zinc, said method comprising passing an electric currentfrom a suitable anode to a cathode to be plated, through an aqueous bathconsisting essentially of a Z-mercaptoethanol salt of the metal to bethus deposited.

References Cited by the Examiner UNITED STATES PATENTS 1,857,507 5/1932Hichman et al. 20446 2,113,517 4/1938 Powell et al. 20446 2,495,6681/1950 Wilson et al 204-52.l

FOREIGN PATENTS 690,978 4/ 1953 Great Britain.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

1. AN IMPROVED ELECTROPLATING BATH CONSISTING ESSENTIALLY OF AN AQUEOUSSOLUTION OF A METAL SALT OF A COMPOUND OF THE FORMULA